1. Field of the Invention
This invention relates to a color toner for developing electrostatic images, a process for its production, and a color image forming method.
2. Related Background Art
For electrophotography, a large number of methods are known in the art as disclosed, for example, in U.S. Pat. No. 2,297,691. In general, in this aspect of electrophotography, an electrostatic latent image is formed on a photosensitive member, utilizing a photoconductive material and according to various means, and subsequently the latent image is developed using the toner to form a toner image. The toner image is transferred to a transfer medium such as paper if necessary, and then the toner image thus transferred is fixed to the transfer medium by heating, pressing, heat-pressing or using solvent vapor. A copy is thus obtained.
Various methods have been hitherto proposed as methods for developing latent images using toners and methods for fixing toner images, and methods suited for their respective image forming processes are employed.
Toners used for such purposes have been commonly produced by melt-kneading a thermoplastic resin and a colorant comprising a dye and/or a pigment to uniformly disperse the colorant in the thermoplastic resin, followed by cooling, pulverization and classification to obtain a toner having the desired particle diameters.
This production process (a pulverization process) can produce reasonably good toners, but has certain kinds of limitations, for example, a limitation to the range of selecting toner materials. For example, dispersions of resins with colorants must be brittle enough to be pulverizable by an economically usable production device. Since the dispersions must be made very brittle, groups of particles having a broad range of particle diameter tend to be formed when actually pulverized at a high speed. In particular, a problem may arise such that particles excessively pulverized tend to be included in such groups of particles in a relatively large proportion. Moreover, materials with such a brittleness tend to be further pulverized or powdered when actually used for development in image forming apparatus such as copying machines.
In the pulverization process, it is not easy to uniformly disperse fine solid particles such as colorants in resins. An increase in fog and a decrease in image density may be caused depending on the degree of dispersion of such fine solid particles, and hence great care must be taken. Colorants coming free rupture cross-sections of resin particles colored with the colorants which may cause variations in developing performance of toners.
Meanwhile, to overcome the problems in the toners produced by pulverization, processes for producing toners by suspension polymerization are proposed (Japanese Patent Publication No. 36-10231, British Patent No. 1,583,564, U.S. Pat. No. 4,592,990 and U.S. Pat. No. 4,609,607, etc.). In this suspension polymerization, a monomer composition is prepared by uniformly dissolving or dispersing a polymerizable monomer and a colorant (optionally together with a polymerization initiator, a crosslinking agent, a charge control agent and other additives), and thereafter dispersing the monomer composition by means of a suitable stirrer in a continuous phase (e,g, an aqueous phase) containing a dispersion stabilizer, to cause polymerization to simultaneously take place to obtain toner particles having the desired particle diameters.
The process for producing toners by suspension polymerization enables encapsulation of a low-melting material such as wax into toner particles and does not require the step of pulverizing resins. Hence, the process has the advantages that the energy to be used during the production of toners can be saved and also the step of classifying toner particles can be omitted.
In the process for producing toners by pulverization, it is possible to use pigments of dyes having a polymerization inhibitory action. However, in the process for producing toners by suspension polymerization, some colorants exhibit a remarkable polymerization inhibitory action, and it is important to select proper colorants.
As a method for preventing or prohibiting the polymerization inhibitory action of colorants, Japanese Patent Application Laid-open No. 2-275964, corresponding to U.S. Pat. No. 5,130,220, discloses a method in which a dye or pigment having a polymerization inhibitory action is treated by bulk polymerization, followed by suspension polymerization to produce a toner. According to this method, toner particles can be formed by suspension polymerization while preventing or prohibiting the polymerization inhibitory action of the dye or pigment. However, it is still,sought to provide a process for producing toners by suspension polymerization that can produce a toner having a higher coloring power, a superior triboelectric chargeability and a sharp particle size distribution.